An aircraft fuselage is normally formed by a skeletal frame comprised of elongated frame members including a plurality of curved ribs, formed from ring segments, and a plurality of longerons, formed of substantially straight struts which extent along the longitudinal axis of the fuselage and secure the ribs one to one another. These elongated frame member can have a variety of cross-sectional shapes and configurations, as shown in FIGS. 1A-1G. A skin of aluminum or other sheet material is then attached to the assembled skeletal frame by riveting the skin to an outer radial surface of ring segments forming each rib as well as an outer surface of the longerons connecting the ribs.
To facilitate riveting of the skin to the skeletal frame, sets of pilot holes, each comprising a row of holes having a given arrangement and spacing, are drilled in accordance with a precise predetermined hole pattern. The row of pilot holes extend along the longitudinal axis of the elongated frame member and are provided at the outer surface thereof. Thereafter, the skeletal frame is assembled, the pilot holes are back drilled to size and rivets or other suitable fasteners are passed through the skin and into the holes formed in the frame members to secured the skin to the skeletal frame.
Due to the stresses encountered during the operation of the aircraft, it is necessary that the fuselage have a high degree of strength and structural integrity. This strength is provided by attaching the aircraft skin to the skeletal frame of the fuselage utilizing as many fasteners as possible in close proximity to one another. As a result, each skeletal frame member is typically provided with more than one set or row of holes, each row of holes being arranged relative to one another so as to preferable form a regular pattern of orthogonal rows and columns. In this regard, each set of holes provided in a given frame member need not have the same hole pattern, however, a predetermined relationship between each set of holes does exist, the tolerances of which are extremely small.
Typically, a large aircraft fuselage requires over a hundred ring segments and struts, each ring segment being between five and twenty feet in length and having up to four rows with two hundred holes per row. Also, because aircraft fuselages have a variable geometry from front to rear, the ribs of the fuselage frame are often constructed using ring segments of different sizes, i.e., the ring segments forming each rib of the fuselage frame can have different radius of curvature.
Due to the large number of holes required to attach the skin to the skeletal frame of the fuselage, such construction is presently very labor intensive. In addition, it is also very important that the spacing between the holes and the linear uniformity of such spacing be extremely precise because the improper placement of a single pilot hole can result in the scrapping of an entire workpiece in which many man hours of labor have been invested.
A system is known for drilling sets of pilot holes in the ring segments and struts forming the fuselage frame. This system employs a template, which is properly located and secured to the upper outer surface of the workpiece, the template having a series of notches along one edge which is formed in accordance with the predetermined hole pattern to be drilled. A drilling arrangement, employing a pneumatic drill positioned adjacent to a carriage or track which supports the template and workpiece combination, has an locating arrangement (spring biased tooth or the like) for engaging the notches of the template and for properly positioning the drill related to the workpiece so that each of the pilot holes is drilled at the intended drill site.
In current practice, the operator slides the workpiece/template combination along the carriage until the tooth of the locating arrangement engages one of the notches provided by the template at each of the intended hole sites. The operator then presses a trigger mechanism which causes the drill bit to extend to drill the pilot hole. This operation is repeated until all the pilot holes of the hole set have been drilled.
Best results are obtained by the operator developing a rhythm when sliding the workpiece along the carriage. Inaccuracies can develop because of wearing of the template over time, i.e, the engagement of the tooth into the notch can deform the notch which results in undesirable play and inaccurate placement of the pilot holes. Worn templates must then be replaced at significant expense.
Additionally, the quality of the results obtained with the above-noted system is very dependent upon the skill of the operator. The slightest bounce or wiggle can result in a hole being drilled where it should not be. Also, because only one horizontal row can be drilled per pass, the height of the drill must be adjusted and the process repeated for each row in order to drill multiple rows. This, more often than not, results in columns of holes which are irregular and not aligned, i.e., non-orthogonal. Beyond the inherent inaccuracies of this method, it is also a monotonous procedure, resulting in inattention by the operation which leads to a large number of scrap parts and a further lowering of productivity.